There have been no lack of attempts in the past to improve the properties of various ethylene homopolymers and ethylene copolymers by adding additives and/or by means of chemical reactions.
U.S. Pat. No. 4,147,740 describes a process for the preparation of modified grafted polyethylenes in which an ethylene homopolymer is mixed with from 0.1 to 1.5% by weight, based on the ethylene homopolymer, of an unsaturated polycarboxylic anhydride, in particular maleic anhydride, and the components are reacted at from 140.degree. C. to 210.degree. C. as a melt in the presence of an organic peroxide having a half life of at least 60 seconds.
U.S. Pat. No. 4,506,056 relates to a process for the preparation of modified, grafted polymers with a reduced degree of crosslinking in which maleic anhydride is grafted on the one hand onto an ethylene homopolymer as the base polymer and on the other hand onto an ethylene vinyl acetate copolymer as the base polymer. The reaction is carried out in the presence of a free radical-forming initiator and an additive which suppresses the homopolymerization of maleic anhydride.
Neither U.S. Pat. No. 4,147,740 nor U.S. Pat. No. 4,506,056 gives any indication of the use of ultrahigh-molecular-weight polyethylene for the preparation of modified graft copolymers.
Ultrahigh-molecular-weight polyethylene means an ethylene homopolymer having a very high degree of polymerization and prepared under low-pressure synthetic conditions using Ziegler catalysts. Its molecular weight, measured viscosimetrically, is at least 1,000,000 g/mol. A number of technically important properties, such as notched impact strength, heat resistance, tear strength at elevated temperature, and wear resistance, improve with increasing molecular weight. This combination of features allows ultrahigh-molecular-weight polyethylene to be used in areas where high demands are made of the material used with respect to stress-and load-bearing capacity, not only at high temperatures, but also at low temperatures. However, the lack of adhesive strength of ultrahigh-molecular-weight polyethylene proves to be disadvantageous in the surface coating of moldings.
The ethylene copolymers present in the base polymer mixture are copolymers containing from 10% to 50% by weight of vinyl acetate or from 10% to 50% by weight of methyl acrylate in addition to ethylene and having a melt flow index MFI (190/2.16) of from 0.5 to 50 g/10 min. They are distinguished by properties such as adhesive power or toughness, in particular by an adhesive strength which makes them suitable for a number of applications, for example as adhesion promoters, binders and adhesives. However, they are still in need of improvement with respect to melt flow index and adhesive strength.
There is thus a demand for a substance which has, on the one hand, essentially the properties of ethylene-vinyl acetate copolymers or ethylene-methyl methacrylate copolymers and, on the other hand, has mechanical properties, for example, abrasion resistance and reduced melt flow index, which are improved compared with these copolymers and, at the same time, has increased adhesion and can be prepared in a technically simple way.